153 lines
5.2 KiB
C
153 lines
5.2 KiB
C
/*
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* Copyright (c) 1993 Martin Birgmeier
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* All rights reserved.
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*
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* You may redistribute unmodified or modified versions of this source
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* code provided that the above copyright notice and this and the
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* following conditions are retained.
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*
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* This software is provided ``as is'', and comes with no warranties
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* of any kind. I shall in no event be liable for anything that happens
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* to anyone/anything when using this software.
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*/
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/*
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FUNCTION
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<<rand48>>, <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>, <<mrand48>>, <<jrand48>>, <<srand48>>, <<seed48>>, <<lcong48>>---pseudo-random number generators and initialization routines
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INDEX
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rand48
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INDEX
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drand48
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INDEX
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erand48
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INDEX
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lrand48
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INDEX
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nrand48
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INDEX
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mrand48
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INDEX
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jrand48
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INDEX
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srand48
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INDEX
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seed48
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INDEX
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lcong48
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SYNOPSIS
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#include <stdlib.h>
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double drand48(void);
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double erand48(unsigned short <[xseed]>[3]);
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long lrand48(void);
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long nrand48(unsigned short <[xseed]>[3]);
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long mrand48(void);
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long jrand48(unsigned short <[xseed]>[3]);
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void srand48(long <[seed]>);
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unsigned short *seed48(unsigned short <[xseed]>[3]);
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void lcong48(unsigned short <[p]>[7]);
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DESCRIPTION
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The <<rand48>> family of functions generates pseudo-random numbers
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using a linear congruential algorithm working on integers 48 bits in size.
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The particular formula employed is
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r(n+1) = (a * r(n) + c) mod m
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where the default values are
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for the multiplicand a = 0xfdeece66d = 25214903917 and
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the addend c = 0xb = 11. The modulo is always fixed at m = 2 ** 48.
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r(n) is called the seed of the random number generator.
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For all the six generator routines described next, the first
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computational step is to perform a single iteration of the algorithm.
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<<drand48>> and <<erand48>>
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return values of type double. The full 48 bits of r(n+1) are
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loaded into the mantissa of the returned value, with the exponent set
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such that the values produced lie in the interval [0.0, 1.0].
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<<lrand48>> and <<nrand48>>
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return values of type long in the range
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[0, 2**31-1]. The high-order (31) bits of
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r(n+1) are loaded into the lower bits of the returned value, with
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the topmost (sign) bit set to zero.
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<<mrand48>> and <<jrand48>>
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return values of type long in the range
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[-2**31, 2**31-1]. The high-order (32) bits of
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r(n+1) are loaded into the returned value.
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<<drand48>>, <<lrand48>>, and <<mrand48>>
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use an internal buffer to store r(n). For these functions
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the initial value of r(0) = 0x1234abcd330e = 20017429951246.
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On the other hand, <<erand48>>, <<nrand48>>, and <<jrand48>>
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use a user-supplied buffer to store the seed r(n),
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which consists of an array of 3 shorts, where the zeroth member
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holds the least significant bits.
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All functions share the same multiplicand and addend.
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<<srand48>> is used to initialize the internal buffer r(n) of
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<<drand48>>, <<lrand48>>, and <<mrand48>>
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such that the 32 bits of the seed value are copied into the upper 32 bits
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of r(n), with the lower 16 bits of r(n) arbitrarily being set to 0x330e.
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Additionally, the constant multiplicand and addend of the algorithm are
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reset to the default values given above.
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<<seed48>> also initializes the internal buffer r(n) of
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<<drand48>>, <<lrand48>>, and <<mrand48>>,
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but here all 48 bits of the seed can be specified in an array of 3 shorts,
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where the zeroth member specifies the lowest bits. Again,
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the constant multiplicand and addend of the algorithm are
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reset to the default values given above.
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<<seed48>> returns a pointer to an array of 3 shorts which contains
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the old seed.
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This array is statically allocated, thus its contents are lost after
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each new call to <<seed48>>.
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Finally, <<lcong48>> allows full control over the multiplicand and
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addend used in <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>,
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<<mrand48>>, and <<jrand48>>,
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and the seed used in <<drand48>>, <<lrand48>>, and <<mrand48>>.
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An array of 7 shorts is passed as parameter; the first three shorts are
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used to initialize the seed; the second three are used to initialize the
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multiplicand; and the last short is used to initialize the addend.
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It is thus not possible to use values greater than 0xffff as the addend.
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Note that all three methods of seeding the random number generator
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always also set the multiplicand and addend for any of the six
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generator calls.
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For a more powerful random number generator, see <<random>>.
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PORTABILITY
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SUS requires these functions.
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No supporting OS subroutines are required.
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*/
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#include "rand48.h"
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void
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__dorand48 (struct _reent *r,
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unsigned short xseed[3])
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{
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unsigned long accu;
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unsigned short temp[2];
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_REENT_CHECK_RAND48(r);
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accu = (unsigned long) __rand48_mult[0] * (unsigned long) xseed[0] +
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(unsigned long) __rand48_add;
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temp[0] = (unsigned short) accu; /* lower 16 bits */
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accu >>= sizeof(unsigned short) * 8;
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accu += (unsigned long) __rand48_mult[0] * (unsigned long) xseed[1] +
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(unsigned long) __rand48_mult[1] * (unsigned long) xseed[0];
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temp[1] = (unsigned short) accu; /* middle 16 bits */
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accu >>= sizeof(unsigned short) * 8;
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accu += __rand48_mult[0] * xseed[2] + __rand48_mult[1] * xseed[1] + __rand48_mult[2] * xseed[0];
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xseed[0] = temp[0];
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xseed[1] = temp[1];
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xseed[2] = (unsigned short) accu;
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}
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